1. Field of the Invention
The invention relates to a micro-evaporator according to the preamble of claim 1
2. Description of Related Art
Micro-evaporators are used to evaporate liquid media such as water, alcohols or alcohol/water mixtures, liquid gases or liquid alkanes for further processing. Such micro-evaporators are for example used in the fields of fuel cell engineering.
Different designs of evaporators are known. So-called plate heat exchangers consist of corrugated steel plates provided with channels for transferring the liquid like filter presses. Resistant rubber seals hold the plates at a distance from each other and seal the channels of the two fluids from each other. Due to the rubber seals, the plate heat exchangers are restricted to a maximum temperature of 250° C. The two fluids flow in a concurrent or a countercurrent flow as thin layers alternating up or down through a series of chambers and contact each other on both chamber walls. Corrugated plate profiles increase turbulence and improve the transfer of heat. Such evaporators are not compact and, when they are operated with heating gas, they have low power densities. Such plate heat transfer systems are for example known from Vauck/Muller; Grundoperationen der Verfahrenstechnik; Leipzig 1994.
An evaporator with micro-structured components for the partial or complete evaporation of liquids is for example known from U.S. Pat. No. 6,126,723. The reactor consists of two fluid chambers for a first and second fluid, and the fluid chambers are connected by means of plates that are porous or provided with micro-structured holes. The reactor is intended for transferring a working fluid contained in a first liquid into a second fluid, and the plates provided with micro-structured holes are only permeable to the working fluid. This reactor can also be used as an evaporator.
Micro-structured devices for conducting media are known from DE 199 63 594 A1 and DE 100 10 400 C2. These devices are especially suitable for evaporating liquid media.
The device in DE 199 63 594 A1 has a layered structure, and at least one layer possesses a number of micro-channels through which the medium to be evaporated flows. A second layer also has a number of micro-channels through which a heat-transferring medium flows. The micro-channels each have an inlet and outlet. To satisfactorily evaporate the liquid medium, the outlets of the micro-channels have a smaller area for the medium to be evaporated and/or have a different geometrical structure than the respective inlets. The pressure of the liquid medium is thereby increased in the micro-channels so that overheated, still-liquid medium suddenly transitions from a liquid to a vapor after leaving the smaller area outlets.
According to DE 100 10 400 C2, the heat output of the heating device is adjustable at least in areas independent of other areas to adjust a desired temperature profile on the surface of the flow channels, at least in individual areas of the flow channels in the direction of flow.
According to this state-of-the-art, the micro-channels for guiding the liquid to be evaporated are parallel. The disadvantage of this micro-evaporator is that the vapor bursts or vapor bubbles can arise if the optimum temperature range is not adjusted. The optimum operating point must be maintained which requires several parameters to be harmonized such as the flow speed and pressure of the medium to be evaporated, the heat output, etc. Such micro-evaporators therefore malfunction easily and cannot be flexibly used.
The problem of the invention is to provide a micro-evaporator that is easier to handle and can be operated problem-free over a wider temperature range